Sound absorption, Thermal and Mechanical behavior of

International Journal of Scientific & Engineering Research, Volume 4, Issue 5, May-2013 301 ISSN 2229-5518

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Sound absorption, Thermal and Mechanical behavior of Polyurethane foam modified with

Nano silica, Nano clay and Crumb rubber fillers R.Gayathri, R.Vasanthakumari, C.Padmanabhan

ABSTRACT

Lot of research is going on in developing materials suitable for absorbing sound and reducing noise. By virtue of their superior vibration damping

capability and attractive characteristics such as visco elasticity, simple processing and commercial availability polyurethane foams are extensively

applied not only in automotive seats but also in various acoustical parts. However, the sound absorption coefficient of polyurethane foams is high (0.8 –

1.0) in high frequencies ranging from 300 to 10000Hz while it is found to be low (0 to 0.5) at low frequencies (10 to 200 Hz).

In this study new polyurethane based porous composites were synthesized by in situ foam rising polymerization of polyol and diisocyanate in the

presence of fillers such as nano silica, crumb rubber and nano clay. The effect of these fillers at various concentrations up to 2% was studied on sound

absorption characteristics, thermal stability, and mechanical properties. Sound absorption coefficient was determined using standing wave sound

impedance tube method. The sound absorption coefficient of filled PU foams is found to be increasing from 0.5 to 0.8 with increasing frequency from

100 to 200 Hz at higher content of the fillers employed. In addition to enhanced sound absorption properties in low frequency region, the composite

foams exhibit superior thermal and mechanical properties. Further foam cell structure and size determined by using SEM and its effect on various

properties will also be highlighted.

Index Terms - crumb rubber, low frequency sound, nano clay, nano silica, Polyurethane foam, Sound absorption coefficient.

—————————— —————————— 1. INTRODUCTION

Now a day the noise pollution has become a serious issue, the

demand for a better environment and more diversified life styles

is increased. Therefore thin, light weight and low-cost composite

materials that will absorb sound waves in wider frequency range

are strongly desired. Polymeric foams have been widely used as

sound absorbing materials and sound energy of incident sound

wave falling on the material is partially dissipated as heat due to

air friction inside polymeric cells and viscous friction between

adjacent polymer chains [1].

R.Gayathri Research Scholar, Dept. of Polymer Tech, B.S.Abdur Rahman

University, Chennai. Mail Id : [email protected].

R.Vasantha kumari, Professor, Dept. of Polymer Tech, B.S.Abdur Rahman

University, Chennai. Mail Id: [email protected].

C.Padmanabhan Professor Dept of Mechanical Engg IIT Madras.

Mail Id: [email protected].

Flexible polyurethane (PU) foams have been extensively used for

absorbing sound and reducing noise, whose attractive

characteristics include its excellent visco elasticity, relative

simple processing, light weight and commercial availability. They

are used as seating, cushioning and sound absorbing material in

automobile industry and as sound absorbers in compressors,

pumps, boilers, electrical installations etc [1].

Generally the sound absorption capacity of PU is strong in high-

frequency regions but relatively weak in low-frequency because

of the low capacity of sound energy attenuation [2]. The sound

absorption ability of polymeric foams is critical especially for the

low-frequency noise. Materials with greater thickness are needed

to achieve good sound absorption at lower frequency region. A

large portion of the structural-borne noise occurs in low frequency

in the range of 30-500 Hz while air-borne noise is mostly

contained in medium and high frequency ranges of 500-8000 Hz

[3]. Possible sources of low frequency noise are many and varied

but are often industry related such as pumps, compressors,

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generators etc. Apart from PU foam system there are other lot of

systems and fillers were studied for sound absorption.

Mendelssohn et al [4] studied the hollow porous microspheres of

polystyrene dispersed randomly in PU, and the obtained material

has many properties, including high porosity, high compression

strength, low acoustic reflectivity, and relative intensity to the

changes of the frequency. Cushman et al [5, 6 and 7] found that

the mixtures of high and low characteristic acoustic impedance

fillers loaded in the polymer can reduce the noise generated by

sound, vibration, and shock, and the obtained material has

excellent sound absorption properties. Verdejo et al[8] found that

low loading fraction of carbon nanotubes (CNT’s) in flexible

polyurethane foams have relatively high effect in sound

absorption; even 0.1% CNT’s can enhance the acoustic absorption

dramatically, which leads the peak absorption coefficient to

increase up to 90% from 70% for the pure polymer foam

especially in the high frequency region. Recent researches on

recycled rubber particles from tyre known as crumb rubber shows

that crumb rubber can be employed as filler for noise absorption

study [9 - 11]. Jamaluddin N et al showed that multi-layer coconut

coir fibres with airspace layers increase the absorption coefficient

of the material at lower frequencies [12]. Sezgin Ersoy et al

suggested that the backing of industrial tea-leaf-fibre with a single

layer of cotton cloth increases its sound absorption properties

significantly [13]. Yang HS et al showed that Composite boards

of random cut rice straws and wood particles, were found to

demonstrate higher sound absorption coefficient than

particleboard, fiberboard and plywood [14]. In order to get many

desired properties in single system nano composites has been

adapted widely. Acoustic properties of PU foams are usually

improved by incorporation of micro-sized fillers because higher

density and better morphology can be achieved but high amounts

of micro fillers can lead to increase in weight of foam and

reduced sound absorption efficiency. Hence studies were carried

out with nano materials which can lead to significant

improvements in sound absorption without much negative effects,

especially in weight increasing [15, 16].

The present research work deals with the preparation and

properties of flexible PU foam filled with three different fillers

namely Nano Silica (NS), Crumb Rubber (CR) and Nano Clay

(NC) at different composition to study their effect on sound

absorption at low frequency range. The effect of these fillers on

thermal and mechanical properties is also highlighted.

2. EXPERIMENTAL 2.1 Materials The commercial raw materials of PU foam, including Part A (the

mixture of polyether polyol, catalyst, blowing agent and

surfactant) and Part B (isocyanate based on mixture of TDI and

MDI), were supplied by Manali Petrochemicals ltd, Chennai.

Nanosilica with trade name Cab-o-sil was supplied by Cabot

Corporation, Chennai. 40 mesh size crumb rubber was supplied

by RK Polymers, Chennai and Nano clay (Organically modified

Montmorillonite clay ,OMMT) with trade name Nanofil 5 was

supplied by Sud Chemie, Germany. Following literature studies

and considering the limitations in the preparation of the

isocyanate mixture with fillers [2] the following quantities of

fillers were used in this study 0.35%, 0.70%, 1.4% and 2.0% .

2.2 Method The PU foams with and without varied content of fillers were

prepared by the free rising foaming method. The desired amount

(0 - 2%) of each filler was mixed with isocyanate (Part B) using a

magnetic stirrer for 30 min. Then, Part A was added (with mass

ratio of 100:38 for Part A and Part B) and stirred with a

mechanical stirrer at 1500 rpm for 15 Sec. The mixture was then

poured rapidly into an open cylindrical mould of dimension

100mm dia before foaming starts. It was allowed to cure at room

temperature for 12 hours and then demolded.

2.2.1 Physical and Mechanical measurements Foam density measurements were carried out as per IS 7888-1976

standard. Average of 5 values of density was considered for each

sample. Universal Testing Machine (UTM, DAK Series 9000)

was used to determine the tensile strength and elongation at break

of all samples at room temperature as per IS 7888-1976 standard.

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The cross head speed was kept as 500mm/min. Uniaxial

compression tests were carried out in UTM, DAK Series 9000

according to EN ISO 3386-1 standard. All the compression test

measurements were performed at a crosshead speed of

100mm/min.

2.2.2 Microscopic studies The surface micro structure was observed using S-3400 Scanning

Electron Microscope (SEM) for pure and filled PU samples after

vacuum sputter coating with gold.

2.2.3 Thermo gravimetric analysis (TGA) TGA studies were carried out using SII Nanotechnology

instrument TG / DTA 6200 with ~ 10 mg sample up to 800 oC at a

heating rate of 20 oC / min in nitrogen atmosphere.

2.2.4 Sound Absorption Coefficient measurement The sound absorption test was carried out at IIT Madras using

Standing Wave Apparatus. The acoustic test system comprises of

an impedance tube, microphone, loud speaker and digital

frequency analyzer as shown in Fig.1. The absorption coefficient

was calculated as the average value of three cylindrical foam

pieces of dimension 90mm in diameter and 15 mm thick, for

different frequencies in the range from 100 to 200 Hz. Sound

absorption coefficient( α ) can be defined as the ratio of energy

absorbed by a material to the energy incident upon its surface.

Fig.1 Standing wave apparatus

Theory

Assuming that a pipe of cross-sectional area S and length L is

driven by a piston at x=0. If the piston vibrates harmonically at a

frequency sufficiently low that only plane waves propagate.(Fig.

2) For a circular waveguide (pipe) filled with air, the highest

frequency at which only plane waves will propagate is given by

fmax =100/ a where ‘a’ is the radius of the waveguide. When the

pipe is terminated with acoustic absorbing material, some of the

incident sound energy is absorbed by the material and the

reflected waves do not have the same amplitude as incident

waves. In addition the absorbing material introduces a phase shift

upon reflection. The amplitude at a pressure anti-node (maximum

pressure) is A+B, and the amplitude at a pressure node (minimum

pressure) is A-B. It is not possible to measure A or B directly.

However, the amplitude at a pressure node and anti-node can be

measured using a microphone probe which is set in a standing

wave tube. We define the ratio of pressure maximum to pressure

minimum as the standing wave ratio (SWR).

Thus SWR = (A + B) / (A - B) where A+B is Pressure

maximum, A-B is pressure minimum.

The reflection coefficient R is defined by

R = B / A, = (SWR + 1) / (SWR - 1)

and finally the Sound absorption coefficient α = 1 - R2 = 1 – (SWR - 1)2 / (SWR + 1)2

Fig. 2 Propagation of sound waves

3.0 RESULTS AND DISCUSSION

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3.1 Foam density and Microstructure: The densities of the filled foams (Fig.3) are higher than that of the pure polymer foam. This may be due to high content of fillers which would fill up more voids thus increasing the density.

Fig. 3 Effect of fillers on density Microstructure of the PU samples was determined using SEM and

Fig 4 (a) to (d) show the SEM images of pure PU foam and filled

PU foam with 1.4 % nano silica, 1.4% crumb rubber and 1.4%

nanoclay respectively.

Fig. 4a Pure foam

Fig. 4b 1.4%Nano silica in PU foam

Fig. 4c 1.4% Crumb rubber in PU foam

Fig. 4d 1.4% Nano clay in PU foam

Cell edges and cell walls are distinctly visible with almost

uniform cell structures throughout in all the compositions of PU

foams. Close inspection of polymer matrix reveals a good

dispersion of the fillers thorough out the sample, in both the walls

and particularly the strut of cellular structure [8].

Table 1 Mean cell size and mean cell wall thickness of pure foam and 1.4 % filled PU foams.

SEM results are further analysed for cell dimensions and the

results are shown in Table 1. Both cell size and cell wall thickness

of filled foams are higher than that of pure foam. Increase in cell

size may be attributed to increased gas diffusion. One hypothesis

is that diffusion is enhanced at the polymer/filler interface due to

poor interaction and increased free volume in the polymer [8].

3.2 Thermal Stability

S. No

Properties Pure foam

Nano silica

Crumb rubber

Nano clay

1 Mean cell size (µm)

269 274 278 272.3

2 Mean cell wall thickness (µm)

91.95 97.6 102.15 93.5

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Temp Cel800.0700.0600.0500.0400.0300.0200.0100.0

TG %

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

1.4% NS

1.4% CR

1.4% NC

Pure PU foam

One of the draw backs of PU foam is its poor thermal stability. So

TGA was performed to assess the effect of the addition of nano

silica, crumb rubber and nano clay fillers on the thermal stability

of the flexible polyurethane foam.

Fig. 5 TGA thermo grams PU foam with and without 1.4% filler

Fig. 5 shows the thermo grams of the foam samples.

Table 2 summaries the results of thermal stability at 50%

decomposition of the samples obtained from the TGA thermo

grams. The temperature of 50% mass loss corresponds to the

temperature range of the decomposition of hard segments. The

value of this temperature increased with the presence of each

investigated filler [17]. The maximum increase was observed at

1.4% loading levels of NS, CR and NC (Fig.5). The residual mass

remaining at 600o C is 8.8% for pure PU foam, 9.3% for NS / PU

foam, 5.4% for CR / PU foam and 11.9% for NC / PU foam.

Table2 Thermal stability of pure and filled foams

3.3 Mechanical properties The effect of fillers on tensile properties of PU foam is shown in

Fig 6a and 6b. As expected, there is a gradual increase in tensile

strengt

h and

decrea

se in

elonga

tion at

break

with

increas

e in

filler content for all the three fillers. The increase in cell wall

thickness with the addition of fillers makes the cell wall stiff

and results in a reinforcing effect on PU foam [3].

Filler percent

Nano silica

Crumb rubber

Nano clay

0% 378.2 378.2 378.2

0.35% 378.6 380.9 390.3

0.70% 392.6 387.9 393.0

1.4% 396.4 393.4 399.6

2.0% 395.3 390.5 394.0

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Fig.6a

Fig. 6b

Effect of fillers on tensile strength (6a) and elongation at break (6b)

Compression strength of the samples was measured at 50%

deflection and the compression strength values show an

increasing trend with the increase in filler content (Fig.7). It is

assumed that the fillers, as an additional physical cross linker,

increased the modulus of flexible segment in the polyurethane

matrix resulting in increased compression strength [18].

Compression strength shows a maximum value at 1.4% loading

followed by decrease at 2.0%. Higher amounts of fillers beyond

1.4% make the cell wall brittle, resulting in decreased

compression strength.

Fig. 7 Effect of fillers on compressive strength (Fig.7)

3.4 Sound absorption of pure and filled flexible polyurethane foams

PU samples with 0.35%, 0.70%, 1.4% and 2.0% of NS, CR and

NC were tested at the frequency ranging from 100 to 200 Hz in

the experiment. Fig 8a, 8b and 8c show the experimental results

for the acoustic absorption coefficient of the samples, as a

function of frequency.

Fig. 8a PU foam with NS

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Fig. 8b PU foam with CR

Fig. 8c PU foam with NC

Fig. 8d PU with 1.4% NS, CR and NC

From these figures it is clear that absorption coefficient increases

with increase in filler content and with increase in frequency. Pure

foam shows an increase in the absorption coefficient only up to

52% in the frequency range 100 – 200 Hz. On the other hand the

addition of various fillers shows an increase up to 80%. All the

three fillers at loading level of 1.4 % show superior sound

absorption capacity at low frequency region of 100-200Hz. It is

also found that for the filler content of 1.4% of NS, CR and NC

sound absorption is the highest (Fig. 8d).The increase in acoustic

effect may be due to the large surface area of fillers at the PU-

filler interface where the acoustic energy can be dissipated as heat

energy [8]. Further in case of porous sound absorbers sound

propagation takes place in a network of interconnected pores such

that viscous and thermal interaction causes the acoustic energy to

be dissipated and convert them into heat energy. At low

frequencies porous PU foams absorbs sound by energy loss

caused by heat exchange. This is an isothermal process. The

absorbed acoustic energy moves inside the cells by the friction

with air. This friction is changed into heat. Formation of fine

morphology by fillers creates more paths for passing sound waves

into foam structure and thus, they absorb more sound.

4.0. CONCLUSION

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Filled PU foam composites with different loading levels (0.35%,

0.70%, 1.4% and 2.0%) of nano silica, crumb rubber and

nanoclay were prepared by free rising foaming method. Increase

in filler content affected the foaming process and cellular

structure of foam as studied from SEM pictures. Maximum sound

absorption coefficient of 80% and improved thermal properties

were obtained at 1.4% weight concentration of all the three fillers.

Mechanical properties also show a significant improvement with

the addition of fillers. It is interesting to find that foam thickness

of 15mm is sufficient to result in improvement in acoustic

properties with fillers. Thus, from the above studies one can

conclude that flexible PU foam with 1.4% weight concentration

of nano silica, crumb rubber or nano clay can improve the

acoustic property in lower frequency range 100-200Hz in addition

to enhancement in thermal and mechanical properties. Further

studies are in progress to determine optimum thickness of the

foam for best sound absorption coefficient in the low frequency

range.

ACKNOWLEDGEMENT The support provided by IIT, Madras in determining sound

absorption coefficient of the samples is gratefully acknowledged.

REFRENCES

[1] Joonmo Lee, Gue-Hyun Kim, Chang-Sik Ha, Sound Absorption

Properties of Polyurethane/Nano-Silica Nano composite Foams, Journal of

applied polymer science, 123, 2384-2390, 2011.

[2] Liu Ting, MAO Liangliang, LIU Fuwei, JIANG Wuzhou, He Zhao,

FANG Pengfei, Preparation, Structure, and Properties of Flexible

Polyurethane Foams Filled with Fumed Silica, Wuhan University, Journal of

Natural Sciences, 16, 029-032, 2011.

[3] Chang Hyun Sung, Kyung sick Lee, Kyu Se Lee, Seung Min Oh, Jae

Hoon Kim, Min Seok Kim, Han Mo Jeong. Sound Damping of a Polyurethane

Foam Nanocomposite, Macromolecular Research , 15 no.5 443-448, 2007.

[4] Mendelsohn M A, Bolton R, Nvaish J, Sound absorbing and decoupling

syntactic foam. US: 5422380[P]. 1995-06-06.

[5] Cushman W B, Thomas G B. Acoustic attenuation and vibration damping

materials. US: 5400296[P].1995-03-21.

[6] Cushman W B, Fla P. Acoustic absorption or damping material with

integral viscous damping. US: 5745434[P]. 1998-04-28.

[7] L.J. Gibson,MF . Ashby. Cellular solids: structure and properties, 2nd

ed. Cambridge University, 1997.

[8] Verdejo R, Stampfli R, Alvarez Lainez M, et al. Enhanced acoustic

damping in flexible polyurethane foams filled with carbon nano tubes.

Composites Science and Technology, 69: 1564-1569, 2009.

[9] Zhu Han, Liu Chunsheng, Tom Kombe, Norasit Thong-On, Crumb

rubber blends in noise absorption study. Materials and Structures, 41: 383-

390, 2008.

[10] Zhou Hong, Li Bo, Huang Guangsu, He Jia.A novel composite sound

absorber with recycled rubber particles.Journal of sound and vibration, 304:

400-406, 2007.

[11] J. Zhao, X.-M. Wang, J.M. Chang , Y. Yao , Q. Cui, Sound insulation

property of wood–waste tire rubber composite, Composites Science and

Technology, 70:

2033–2038, 2010.

[12] Nor MJM, Jamaluddin N, Tamiri FM. A preliminary study of sound absorption using multi-layer coconut coir fibres. Techn Acoust Electron J, 3, 2003. [13] Sezgin Ersoy, Haluk Ku¨c_u¨k, Investigation of industrial tea-leaf-fibre waste material for its sound absorption properties, Applied Acoustics, 70, 215–220, 2009. [14] Yang HS, Kim DJ, Kim HJ. Rice straw–wood particle composite for sound absorbing wooden construction materials. Bioresour Technol,86, 117–21, 2003. [15] Ling Y, Yan MX, Xu J, Ming LZ, Hua TJ Polym Degrad Stabil 94:971, 2009. [16] Mello D, Pezzin SH, Amico SC Polym Test 28:702, 2009. [17] Anna Wolska, Marcin Gozdzikiewiez, Joanna Ryszkowska. Thermal

and Mechanical behavior of flexible polyurethane foams modified with

graphite and phosphorous fillers. Journal of Material science, 47:

5627-5634, 2012.

[18] I.Javni, W. Zhang, V.Karajkov, Z.S. Petrovic. Effect of nano and micro

silica fillers on polyurethane foam properties, Journal of cellular plastics, 38:

no:3,229-239,2002.

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